MEMS (microelectromechanical systems) pressure sensors are generally known and widely used. One type of pressure sensor is an absolute pressure sensor which includes a pressure sensing circuit, typically a piezoresistor bridge, formed on the top side of a silicon substrate, and a glass pedestal anodically bonded to the backside of the silicon substrate where a cavity is located to form a reference vacuum. For such an absolute pressure sensor, the front side of the device where the sensing circuit is located faces the pressure media. Many absolute pressure sensors are used in applications in which the sensors are exposed to a harsh media. For such applications, the front side sensing by a traditional absolute pressure sensor cannot survive in the harsh media. These environments require another type of absolute pressure sensor, such as a backside absolute (BSA) pressure sensor, which is resistant to exposure to harsh media.
One typical BSA pressure sensor includes a top cap to enclose a cavity on the front side of the silicon substrate, having the sensing circuit. The cavity contains a reference vacuum and the backside of the sensor is exposed to the pressure media. The reference vacuum is enclosed in a cavity formed by bonding the top cap to the top side of the silicon substrate embedded with a sensing circuit. The backside of the silicon substrate may be bonded with a pedestal having an aperture for accessing the backside of the silicon diaphragm to the pressure media. The silicon diaphragm is formed by selectively removing a portion of bulk silicon from the backside of the silicon substrate.
However, these types of BSA pressure sensors have certain operational drawbacks, such as high cavity pressure due to the residual gasses accumulated during the anodic bonding step for a design and process having limited depth of the cavity of the cap, which may cause high output errors at low/high operating temperature.
Another issue that may occur is a large variation of the cap bonding boundary to enclose the cap cavity. This is because the lateral dimension of the cap cavity is difficult to control while etching the cap cavity. In general, the deeper the depth of the cavity of the cap, the bigger the lateral dimensional error of the cavity of the cap. The lateral dimension of the cap cavity determines the cap bonding boundary with the silicon substrate. The cap bonding boundary is an important factor effecting on the distribution and level of thermal stresses on the sensing circuit. These process errors are often beyond allowed bonding boundary tolerance for balancing thermal stresses, resulting in high output error, which reduces the device yield. Furthermore, these types of sensors are also subject to weak diaphragm strength, and diaphragm damage may result from high pressure fluctuations in different applications.
Accordingly, there exists a need for a BSA pressure sensor which overcomes the drawbacks discussed above.